1. Field of the Invention
This invention relates to video signal processing and, more particularly, to a method and apparatus for detecting the correlation of video information in successive line intervals, thereby indicating the correlation of vertically aligned pixel data.
2. Discussion of the Prior Art
One typical operation in video signal processing is the separation of luminance and chrominance signal components from a composite video signal. Since the respective frequency spectra of the luminance and chrominance components are interleaved, such separation, commonly known and referred to herein as Y/C separation, often is carried out by a comb filter. As is known, however, a comb filter can be used successfully if the signal information in successive horizontal line intervals of the video signal is substantially the same. More particularly, if the video signal, or at least the chrominance components therein, exhibit good vertical correlation, the comb filter will function properly to separate the luminance and chrominance components. However, in the absence of good correlation, that is, if there is little equality in the video information from line-to-line, Y/C separation by means of the comb filter will be erroneous. In that case, separation of the luminance and chrominance components is best achieved by using band pass filters.
Since the information content of a video picture varies, the correlation of a video signal representing the video picture likewise varies. Hence, a typical Y/C separator includes both a comb filter and band pass filters. The selection of the comb filter or the band pass filters is a function of the detected correlation of the video signal; and one or the other is switched into the video signal processing channel to effect Y/C separation.
Another application of correlation detection relates to the avoidance of so-called "cross-luminance". Cross-luminance occurs when the frequency range of the luminance component crosses over and extends into the frequency range of the chrominance component. Since the frequency spectra of the luminance and chrominance components are interleaved, such cross-luminance, that is, an intrusion of a frequency band of the luminance component into a frequency band of the chrominance component, will result in interference between the luminance and chrominance signals. Correlation detection is useful in avoiding such interference.
One technique for detecting vertical correlation of the chrominance component in a video signal compares the video information present in every other horizontal line interval. For example, if the instantaneous horizontal line interval is designated the nth line, the preceding line interval is designated the (n-1)th line and the next preceding line interval is designated the (n-2)th line, correlation is detected by comparing the video information, and particularly the chrominance component, of the nth line to that of the (n-2)th line, such as by subtracting one from the other to produce a difference signal. The chrominance component frequencies included in this difference signal are extracted, as by use of a band pass filter having a pass band corresponding to the frequency band of the chrominance component; and the filtered difference signal thus represents the vertical correlation of the chrominance component. Complete correlation is indicated when the filtered difference signal, that is, the output of the band pass filter, is zero. This indicates that the video information and, particularly, the chrominance information, in the compared line intervals is substantially identical. Conversely, the output of the band pass filter exhibits a maximum level when the information in the compared line intervals is quite different. Thus, it is expected that the signal level at the output of the band pass filter varies between zero and a maximum level, depending upon the vertical correlation of the chrominance component.
It is expected that the aforementioned correlation detecting signal, that is, the output of the band pass filter, will vary significantly over a video field or frame. When correlation is used to select a comb filter or a band pass filter for Y-C separation, it is desirable to avoid frequent switching, if possible. This is achieved by establishing a threshold level to which the correlation detection signal is compared. Correlation is assumed to be present when the correlation detection level is below this threshold level, and it is assumed that there is no correlation when the threshold level is exceeded.
In a conventional correlation detection technique, the vertical spatial frequency characteristics of those components in the chrominance frequency band (i.e. in the vicinity of the chrominance subcarrier frequency) varies as a function of the level of the input signal. Accordingly, the output of the band pass filter, which is used as an indication of vertical correlation, fluctuates as the input signal level changes. Consequently, it is difficult to set a proper threshold level for discriminating between correlation and non-correlation. As an example, if the threshold level is set too high, the band pass filter output may not exceed that high threshold level even though correlation is not present because the input signal level may be relatively low, thus causing the band pass filter output to be below the threshold level. Conversely, if the correlation detection threshold level is too low, the sensitivity of this detecting method becomes such that the threshold level normally is exceeded even when there is relatively good correlation in the video signal. That is, the information in vertically aligned pixels may be quite similar, yet the output level of the band pass filter, although quite low, will exceed the threshold level. As a result, the presence of correlation will be erroneously indicated as non-correlation.
Consequently, cross-luminance avoidance might not be fully effective. Although the conventional correlation detecting technique may operate satisfactorily to avoid cross-luminance for video signals representing special image patterns, such as the color bar pattern, such technique does not satisfactorily avoid cross-luminance for more general picture images.